Using variables in a mongodb query function

Hi I am using Express to work with MongoDB. I am pretty new to both. I got a find function working, it looks like this:

function find (current_collection, cb) {
  getCollection(current_collection, function (err, collection) {
    collection.findOne({
      'identity.sender.id': '1234abcd'
    }, function (err, result) {
      // ... some err and cb handling
    });
  });
}

I logged the result and everything looks fine. I want to this code somewhere else, so I tried to use variables in the query and turned the function into something like this:

function find (current_collection, val, cb) {
  getCollection(current_collection, function (err, collection) {
    collection.findOne({
      'identity.sender.id': val
    }, function (err, result) {
      // ... some err and cb handling
    });
  });
}

// I then called the function on a separate file, like this and it worked

function get_user_data (){
    var x = getModel().find('users', '1234abcd', function(err, result){
    //
    });
    return x;
}

However, when i tried to go one step further but using variable for the query attribute, everything stopped working and i got error:

function find (current_collection, attr, val, cb) {
  getCollection(current_collection, function (err, collection) {
    collection.findOne({
      attr : val
    }, function (err, result) {
      // ... some err and cb handling
    });
  });
}

// I then called the function on a separate file, this time, did not work!!

function get_user_data (){
    var x = getModel().find('users','identity.sender.id', '1234abcd', function(err, result){
    //
    });
    return x;
}

Really not sure what I have done incorrectly. Please help!! Thanks in advance!